The utility and advantages of aqueous ammonium phosphate solutions are well known, particularly in the agricultural industry. Several advantages associated with the presence of acyclic polyphosphates are also recognized. For instance, products obtained from wet-process acids generally contain metallic impurities including iron, magnesium, aluminum and the like, which form unmanageable precipitates upon neutralization of acids containing insufficient polymeric phosphate. Secondly, ammonium phosphate solubility increases in proportion to polymeric phosphate content. Thus, an acid having an H.sub.2 O/P.sub.2 O.sub.5 molar ratio of 3 corresponding to a polyphosphate content of 10 percent based on total P.sub.2 O.sub.5, can be converted directly to the ammonium phosphate solution commercially designated as 8-24-0 containing 8 weight percent nitrogen and 24 weight percent phosphate expressed as P.sub.2 O.sub.5. The more concentrated solution 10-34-0 containing 10 and 34 weight percent nitrogen and P.sub.2 O.sub.5, respectively, requires the use of ammonium phosphate of which at least 50 percent of the P.sub.2 O.sub.5 is polymeric. Similarly, 12-44-0 can be obtained only with ammonium phosphates in which 75 percent or more of the P.sub.2 O.sub.5 is polymerized. Maximum solubilities varies somewhat with pH and temperature, and are obtained under slightly acidic conditions, ie., pH levels below about 7, generally on the order of about 6.5. However, polyphosphate stability increases with pH with the result that pH should be at least about 5, preferably above about 6.
Starting materials include essentially any source of phosphoric acid including wet-process acids, so-called white acids, and the like. The wet-process acids are obtained by acidifying phosphate-containing rock with strong mineral acids such as sulfuric, which convert the calcium or other metal phosphates to phosphoric acid, calcium sulfate, etc. Insoluble sulfates are removed by filtration although the wet-process acids generally contain at least about 1 and often between about 1 and about 20 weight percent cogeneric metallic impurities expressed as the corresponding oxides.
The so-called "white acids" are obtained by the "electric furnace" process in which phosphate-containing rock is reduced by reaction with coke at extremely high temperature generated by electrical current. The phosphate rock is reduced to elemental phosphorus, which is then burned to P.sub.2 O.sub.5 and absorbed in water. While these acids are generally more expensive than wet-process acids, they often become available at prices low enough to justify their use in the manufacture of ammonium phosphates.
The possibility of increasing polymeric content by driving off free and chemically combined waters, i.e., polymerizing the acid, at very high temperatures, has been recognized for some time. Temperatures required to obtain any significant conversion to polymeric P.sub.2 O.sub.5 are at least about 400.degree. F., usually between about 500.degree. and about 750.degree. F.
Earlier attempts at polymerization involved heating the crude acid, i.e., an acid having a relatively high H.sub.2 O/P.sub.2 O.sub.5 ratio, with an external heating source, to obtain the required polymerization, then neutralizing the polymerized product. It was then discovered that the required temperatures could be generated by the autogenous heat of neutralization with ammonia. This procedure accomplished two objectives in one step -- phosphate polymerization and ammonium phosphate or polyphosphate formation. This process could be carried out in either a batch or continuous basis, the latter often involving the use of so-called tubular reactors in which ammonia and the phosphoric acid feed were continuously passed through and contacted in the reactor tube. Numerous variations of both batch and continuous operations have been suggested.
All of these processes require relatively concentrated phosphoric acid feeds in order that the heat of neutralization will be sufficient to reach the required temperatures. For several reasons, the maximum temperatures obtainable by adiabatic neutralization decrease markedly as the H.sub.2 O/P.sub.2 O.sub.5 feed ratio increases. Reaction efficiency, which also depends on several variables, also effects maximum reaction temperature and, consequently, influences conversion, i.e., polymerization level. Even the best designed and controlled reactors can obtain at most 90 percent, and generally less than 90 percent of the theoretical polymer content.
These and other factors make it highly impractical and often impossible to obtain a required polymer content from a given acid feed without preconcentration of the acid to reduce H.sub.2 O/P.sub.2 O.sub.5 ratio prior to neutralization.
It is therefore one object of this invention to provide an improved method for converting phosphoric acids to ammonium phosphates. It is another object to provide a method for producing stable aqueous solutions of ammonium phosphates containing substantial amounts of ammonium polyphosphate. These and other objectives, variations and modifications of the concepts of this invention will be apparent to one skilled in the art in view of the following description, drawings and claims.
Therefore, in accordance with one embodiment, aqueous solutions of ammonium polyphosphates in which at least 25 percent of the P.sub.2 O.sub.5 is present as polyphosphate, are produced by mixing with the feed acid an anhydrous mineral acid selected from sulfuric, nitric and hydrochloric acids, in amounts of at least about 0.05 mole of mineral acid per mole of P.sub.2 O.sub.5 sufficient to produce a liquid phase temperature in the reaction melt of about 400.degree. to about 750.degree. F. upon adiabatic neutralization of the acid mixture with anhydrous ammonia. In another embodiment, similar conversions are obtained with feed acids having equivalent H.sub.2 O/P.sub.2 O.sub.5 mole ratios about about 3.3 and as high as 15.
The amount of ammonia required to obtain this objective is generally on the order of at least about 0.12 weight part of anhydrous ammonia per weight part of the acid mixture, i.e., P.sub.2 O.sub.5 plus mineral acid equivalent. In this disclosure the term "mineral acid" refers only to the additional sulfuric, nitric and/or hydrochloric acids. The term "mineral acid equivalent" refers to the molar equivalent acidity of the mineral acid at the conditions involved, i.e., the extent to which it will react with ammonia.
Reaction conditions are controlled to produce a product melt, using essentially only the autogenous heat of neutralization, at a temperature sufficient to increase the polyphosphate content by at least about 10 percent based on total P.sub.2 O.sub.5 as compared to the feed acid, and form a product of which at least about 25 percent of the P.sub.2 O.sub.5 is present as polyphosphate. The product melt is then quenched and neutralized, preferably to a temperature of about 200.degree. F. or less, generally by immersion in an aqueous medium.